Prosthetic devices for the partial or total replacement of hips, knees and other parts of the human body are widely used. These implant devices are typically formed of materials such as stainless steel, cobalt-chromium-molybdenum alloys and titanium alloys since those materials are biocompatible and exhibit relatively low corrosion rates in body fluids.
For many years, implants were anchored in place through pressure or frictional fit, screws or glue-like materials such as polymethyl methacralate (PMMA). Because of various inadequacies in these fixation techniques, implants were developed that could take advantage of a phenomenon known as tissue ingrowth where bone or soft body tissue would grow into a porous matrix formed on the outer surface of the implant and firmly anchor it in place. Porous surfaces formed of various ceramic, metallic and polymeric materials have been developed.
The advantages of providing an area receptive to tissue ingrowth include providing an implant with a greater interface shear strength between the implant and the bone as well as a more uniform distribution of stresses throughout the implant and surrounding tissue. The goal in the use of such implant devices is to provide for fixation approaching the strength and resilience of natural bone and other tissue.
An example of an early attempt to develop a prosthesis capable of accommodating tissue ingrowth is described in U.S. Pat. No. 3,605,123 where an implant was described as being formed with a cast or wrought substrate and a porous metal overlay. The overlay or coating was applied by a plasma flame spray process, which resulted in a coating with a graduated or uneven porosity from the substrate to the surface. Aside from inadequacies in the use of plasma spray techniques in the formation of porous coatings, a non-uniform porous matrix has been found not to be effective in accommodating tissue ingrowth throughout the matrix resulting in a weaker bond than desired between the implant and surrounding tissue.
Much effort has been directed toward developing an implant with a porous matrix where the pores are substantially uniformly distributed throughout the matrix. One such attempt is described in U.S. Pat. No. 4,206,516 where titanium particles were bonded to a substrate through a sintering process, resulting in a porous coating with an irregular surface, but which had a relatively uniform pore distribution throughout.
A drawback of forming a porous matrix of materials such as titanium as described in U.S. Pat. No. 4,206,516 is that a sintering process normally requires elevated temperatures. For titanium, the microstructure of the naturally occurring mix of fine grained alpha and beta phases is transformed to a pure beta structure when heated to a temperature above about 990.degree. C. (well below the temperature required for sintering), however, upon cooling the beta grains lose their identity (even though their outline remains) and the phases become redistributed such that the desirable physical properties of the metal are lost. Some of these possible recombinations include coarse grained alpha-beta phases and a mix of primary alpha and transformed beta phases. See L. J. Bartlo, Effect of Microstructure on Fatigue Properties of Ti-6Al-4V Bar, American Society for Testing and Materials, 1960, pp. 144-154.
Titanium that has been transformed into and out of the pure beta structure is significantly less desirable than the naturally occurring structure because of a decrease in fatigue strength and ductility. The normal sintering, although capable of providing uniformly distributed pores throughout a matrix, does not result in a pore size range that is controlled closely enough.
U.S. Pat. No. 3,852,045 teaches a porous material and method of production in which an attempt is made to provide better control over the pattern of interconnected pores or voids. The method involves a high energy rate forming process that compacts a metallic powder placed around a former through extremely high pressures created through high energy rate compaction. The compacted element is subjected to a high temperature, vacuum treatment to remove the former and to sinter the material. However, this process also employs sintering temperatures above the level that affects the microstructure of materials such as titanium.